The invention relates to a logging sonde for investigating the geological formations traversed by a borehole.
In order to compare the resistivity at various points on the wall of a borehole, in particular for dip metering, a sonde equipped with measuring pads fitted on their outer faces with electrodes or transducers for being applied against the wall is lowered into the borehole. The sonde is suspended from a cable which also serves to transmit electrical signals between the sonde and surface equipment.
Measurement per se is normally performed while the sonde is progressively raised up the borehole. The pads are then applied against the walls of the borehole by resilient means such as blade springs.
A sonde operating in accordance with this principle is described in U.S. Pat. No. 4,614,250. This sonde includes four measuring pads which are regularly distributed around the body of the sonde with each pad being rigid and made in one piece.
A logging sonde of this type has also been described in the article entitled "Formation Imaging with Microelectrical Scanning Arrays" by M. P. Ekstrom et al, published in The Log Analyst (Vol. 28, No. 3, May-June 1987). The latter sonde includes two adjacent measuring pads 90 degrees apart, for providing an image of the stratigraphic characteristics of the geological formations through which they pass. To this end, each of these two pads is equipped with a distributed array of electrodes such that by vertically displacing the sonde in the borehole it is possible to obtain a complete image over the width of the electrode array. Events of substantially the same size as the electrodes are thus detected in the zone scanned by each pad.
For the sake of economy, logging sondes are generally designed so as to be capable of being used in all existing boreholes, whose diameters vary in practice between 15.2 cm (6 inches) and 50.8 cm (20 inches). The diameter of such sondes after the pads have been retracted along the sonde body must therefore be slightly less than the minimum diameter of a borehole. In practice, the maximum acceptable diameter for a sonde with retracted pads is about 12.7 cm (5 inches).
Because of this size constraint, the angular field of observation of the sonde described in the above mentioned article of M. P. Ekstrom et al is relatively narrow even in a small diameter borehole. The two pads provide two disjoint images in two perpendicular directions with the widths of the images being limited to the widths of the arrays. In practice, the field of observation of this sonde is about 22% in a borehole having a diameter of about 20.3 cm (8 inches). Under these conditions, it can happen that information of major importance for borehole exploitation, e.g. singularities having a vertical or sub-vertical nature in the stratigraphic characteristics of the geological formation passed through, are not detected.
In order to remedy this drawback, it is possible to perform several successive passes along the same borehole with the sonde being angularly offset about its axis from one pass to another.
However, it is rarely possible to obtain the desired result by this operation since boreholes usually have an elliptical section which tends to orient the sonde in a privileged direction which remains the same from one pass to another. Further, the need to run several successive passes of the sonde along the same borehole penelizes the operator since there is a corresponding increase in the time for which the drilling rig is out of service.
An obvious improvement in results would be obtained by equipping a logging sonde with four pads each fitted with an array of electrodes. However, the angular field of observation would remain less than 50% in a borehole having a diameter of about 20.3 cm (8 inches), and this is not enough.